9  Control flow statements: R

9.1 The if - else if - else statement

The if - else if - else statements can check several conditions, and execute the code corresponding to the condition that is true. Note that there can be as many else if statements as required.

Syntax: R uses curly braces {} to identify the code to be executed if a condition is true. All the code in the curly braces within a condition is executed if the condition is true.

Example: Input a number. Print whether it is positive, negative or zero. If it is negative, print its absolute value.

number = readline("Enter a number:")

Enter a number:

number = '3'

number_integer = as.integer(number)
if(number_integer>0)
{
  print("Number is positive")
}else if(number_integer==0)
{
  print("Number is zero")
}else  
{
  print("Number is negative")
}

[1] "Number is positive"

9.2 TryCatch

If we suspect that some lines of code may produce an error, we can put them in a tryCatch() statement, and if an error does occur, we can use the error argument to instead execute an alternative piece of code. In case of warning, we can use the warning argument to instead execute an alternative piece of code. Both the error and warning arguments have a function that is executed in case of errors and warnings respectively. The argument to this function is the error / warning message. The program will direct the code to the relevant function if an error or warning occurs.

num = readline("Enter a number:")

Enter a number:

num = '3r'

tryCatch(
  { num_int = as.integer(num)  ; 
  if(num_int%%3==0)
  {
    print("Number is a multiple of 3")
  }else{
    print("Number is not a multiple of 3")
  }}
  ,warning = function(w) {print(w);print("Input must be an integer")})

<simpleWarning in doTryCatch(return(expr), name, parentenv, handler): NAs introduced by coercion>
[1] "Input must be an integer"

9.3 Loops

9.3.1 for loop

We use the : operator to define a vector of consecutive integers. For example, the sequence of integers from 1 to 10 can be generated with the code 1:10. Usually, we generate a sequence in this manner to iterate over the sequence with a for loop.

Example: Print the first n elements of the Fibonacci sequence, where n is an integer input by the user, such that n>2. In a fibonacci sequence, each number is the sum of the preceding two numbers, and the sequence starts from 0,1. The sequence is as follows:

0,1,1,2,3,5,8,13,….

n = readline("Enter a number:")

Enter a number:

n = 6

#Initializing the sequence to start from 0, 1
n1=0;n2=1

#Printing the first two numbers of the sequence
elements<-c(n1,n2)

for(i in 1:(n-2))  #Since two numbers of the sequence are already printed,n-2 numbers are required 
{
  #Computing the next number of the sequence as the summation of the previous two numbers
  n3 = n1+n2
  elements<-c(elements,n3)
  
  #As 'n3' is already printed, it is no longer the next number of the sequence. 
  #Thus, we move the values of the variables n1 and n2 one place to the right to compute the next number of the sequence.
  n1 = n2
  n2 = n3  
}
print(elements)

[1] 0 1 1 2 3 5

print(paste0("These are the first ", n, " elements of the fibonacci series"))

[1] "These are the first 6 elements of the fibonacci series"

9.3.2 while loop

Example: Print all the elements of the Fibonacci sequence less than n, where n is an integer input by the user, such that n>2. In a fibonacci sequence, each number is the sum of the preceding two numbers, and the sequence starts from 0,1. The sequence is as follows:

0,1,1,2,3,5,8,13,..

n = readline("Enter a number:")

Enter a number:

n = 50

#Initializing the sequence to start from 0, 1
n1=0;n2=1

#Printing the first number of the sequence
elements<-n1
while(n2<n)
{
    #Print the next number of the sequence
    elements<-c(elements,n2)
    
    #Comptuing the next number of the sequence as the summation of the previous two numbers
    n3 = n1+n2
    
    #As n2 is already printed, assigning n2 to n3, so that the next number of the sequence (i.e., currently n3) is printed if the program enters the loop again
    #Assigning n1 to n2 as n1 has already been used to compute the next number of the seqeunce (i.e., currently n3). 
    n1 = n2
    n2 = n3  
}
print(elements)

 [1]  0  1  1  2  3  5  8 13 21 34

print(paste("These are all the elements of the fibonacci series less than", n))

[1] "These are all the elements of the fibonacci series less than 50"